In the planetary gear mechanisms mounted in devices such as the speed reducers, multiple planet gears are meshed with a sun gear while being rotatably supported by a carrier, respectively. With the structure, such a mechanism is formed that the planet gears revolve about the sun gear while rotating about the axes thereof.
For example, Japan Laid-open Patent Application Publication No. JP-A-2009-079627 (disclosed on Apr. 16, 2009) describes a structure of a speed reducer provided with a planetary gear mechanism immersed into lubricating oil. In the planetary gear mechanism, each of planet gears is attached along and onto the outer peripheral surface of a planet shaft (pin) through a bearing. Further, a washer (plate member) and a C-shaped snap ring (retainer portion) are attached onto the upper end portion of the planet shaft for fixing each planet gear onto the planet shaft.